1. Field of the Invention
The present invention relates to an image forming apparatus such as a digital-color copy machine and a method therefor.
2. Description of Related Art
A digital color copy machine is comprised of an image reader for reading a document by a color image reader for reading a document data and a printer which converts gradation data to binary print image signals by binarizing them and prints an image on a copy paper responsive to the print image signals using an electrophotographic method. In the digital color copy machine, the image reading action is performed repeatedly in order to obtain a color copy and the printing action is repeated to a copy paper in every color.
In the printer, a laser diode is driven by print image signals amplified and an image is formed by scanning a laser beam emitted from the laser diode on a photoconductive drum. In a copy machine forming an image using the electrophotographic method, a characteristic of the gradation of an image printed on a copy paper to gradation data of image signals (hereinafter referred to as .gamma.-characteristic) is not linear, as indicated by a reference numeral 81 in FIG. 7(A). In order to obtain a linear gradation characteristic 83 of the printed image, it is necessary to provide a .gamma.-correction circuit for correcting gradation data of a document image using a correction characteristic 82 reversed from the .gamma.-characteristic 81. However, in the conventional copy machine, gradation data as input data to the .gamma.-correction circuit has a bit number same to that of corrected gradation data outputted therefrom. As apparent from FIG. 7(A), the gradient of the correction characteristic 82 in a range of low gradation is smaller than that in a range of higher gradation. Due to this, an error becomes large between the gradation of the correction characteristic 82 and that of digital gradation data having been corrected, especially in a low gradation range and, thereby, it becomes impossible to reproduce a document image faithfully.
Further, in the conventional copy machine, the .gamma.-correction circuit has only one correction characteristic 82 and, when the .gamma.-characteristic of the copy machine is varied by a change in the environment of use such as temperature, humidity and the like the gradation characteristic of a printed image becomes non-linear and this makes it impossible to form an image faithfully.